UV disinfection of biologically active carbon filter effluent

Abstract

The biological activated carbon (BAC) filters at the Central Lake County Joint Action Water Agency (CLCJAWA; Lake Bluff, Ill.) shed heterotrophic plate count (HPC) bacteria, especially when water temperatures exceed 15°C (59°F). Chlorine, previously the only disinfectant applied to BAC filter effluent at CLCJAWA, was augmented with the addition of ultraviolet (UV) reactors in 2004, creating a multiple barrier. An average collimated beam UV dosage of 12 mJ/cm2 was established for 3‐log inactivation of indigenous HPC bacteria, but the equivalent effective reactor dosage was difficult to determine with certainty. This unique UV application reduced 95th percentile BAC filter effluent HPC concentrations from 233 to 3 cfu/mL and the percentage of coliform‐positive samples from 25 to 0.07%. The reactors' medium‐pressure UV lamps remained efficacious at seven times their recommended replacement interval of 3,000 h. This article documents full‐scale HPC and total coliform inactivation in a water plant, describes the use of indigenous HPC concentrations to monitor UV reactor performance, and demonstrates the potential for extended lamp life.